[libvirt] Serial connection between guests

[Martin Kletzander]

On Tue, Jul 14, 2015 at 08:25:27AM -0300, Marcelo Ricardo Leitner wrote:
>Hi,
>
>On Tue, Jul 14, 2015 at 09:52:12AM +0200, Martin Kletzander wrote:
>> On Fri, Jul 10, 2015 at 04:28:46PM -0300, Marcelo Ricardo Leitner wrote:
>> >Hi,
>> >
>> >(I'm not subscribed to the list, please keep me on Cc)
>> >
>> >I'm attempting to get a serial link between two guests, same hypervisor.
>> >The only practical way I could find is to add a serial port using a pty
>> >to a guest and then manually connecting to the serial (console in my
>> >case) of the other guest using socat in the hypervisor.
>> >
>> >Then it made me think.. we could have this implemented at libvirt level.
>> >We could have a serial port on which we choose pty, udp, tcp, etc, and
>> >also a serial port from another guest, so that libvirt would handle
>> >socat start/stop automatically as both guests come up/down. Maybe
>> >libvirt could even do something smarter than that, maybe it can avoid
>> >socat somehow.
>> >
>>
>> You can have one domain with serial port that will listen on some
>> interface (unix socket, ip and port, whatever) and then have the
>> second one connect to it.  That could be done for example like this:
>>
>> XML snippet of device for domain A:
>>  <serial type='file'>
>>    <source mode='bind' path='/tmp/tahi.sock'/>
>>    <target port='1'/>
>>  </serial>
>>
>> XML snippet of device for domain B:
>>  <serial type='file'>
>>    <source mode='connect' path='/tmp/tahi.sock'/>
>>    <target port='1'/>
>>  </serial>
>>
>> This way you need to make sure domain A is started when you are
>> starting domain B, so it can connect to the socket that was created by
>> domain A.  If you don't want to depend on the order of domains being
>
>I'd like to avoid that as much as possible. (though I failed to connect
>the serial interfaces through libvirt with any of the configs, tcp,
>unix, etc..) perhaps due to the console thing you mentioned below.
>
>> started, you can use for example socat for that:
>>
>> XML snippet of device for domain A:
>>  <serial type='file'>
>>    <source mode='bind' path='/tmp/tahi-domA.sock'/>
>>    <target port='1'/>
>>  </serial>
>>
>> XML snippet of device for domain B:
>>  <serial type='file'>
>>    <source mode='bind' path='/tmp/tahi-domB.sock'/>
>>    <target port='1'/>
>>  </serial>
>>
>> And then run socat as:
>>  socat unix:/tmp/tahi-domA.sock unix:/tmp/tahi-domB.sock
>
>That's what I was doing in the end. It also allows me to change the
>connection on the fly if needed, but I have to keep an eye on socat. I
>was thinking if we could have something more automatic for it.
>
>Like, if I have domA and domB connected through a serial link using
>socat, and I reboot the hypervisor, I wanted the socat to come back
>automatically. Same if it dies..
>

You can use hook for that, probably

https://libvirt.org/hooks.html

>Maybe it could be handled like Ethernet network is. You add a cable, and
>on cable specs you define which dom/port is connected to that end.  I
>tried to think on an UI that would allow this but all my ideas are
>confusing.
>
>> For more details on how this works and what you can use, see the
>> documentation [1].  Beware that if it's the only serial port n the
>> guest, it will become a console, so you might want to add it instead
>> of modifying an existing one.
>
>That's probably what kept it from working here for a long time hehe took
>me a while to notice that.
>
>> Also, the examples are done without any security labels in mind.  If
>> you use SELinux, for example, you'll probably need to adjust the
>> <seclabel/> for the sockets, see the docs [2] for details.
>
>Thanks,
>Marcelo
>
>>
>> Martin
>>
>> [1] https://libvirt.org/formatdomain.html#elementCharChannel
>> [2] https://libvirt.org/formatdomain.html#seclabel
>>
>> >What do you think? My usage is for virtualizing TAHI:
>> >http://networktest.sourceforge.net/usage.html
>> >I need 2 ethernet links plus a serial one, which TAHI can't break while
>> >running the tests.
>> >
>> >I didn't think this regarding multi-platform & all.. just sharing the
>> >idea/need.
>> >
>> >Thanks,
>> >Marcelo
>> >
>> >--
>> >libvir-list mailing list
>> >libvir-list at redhat.com
>> >https://www.redhat.com/mailman/listinfo/libvir-list
>
>
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